1. Field of the Invention
This invention relates to a liquid crystal display device of the active matrix type, and more particularly to the structure of a thin film transistor formed as an active element in an integrated circuit for use with a liquid crystal display device of the active matrix type.
2. Description of the Related Art
Thin film transistors (TFTs) have been developed remarkably in recent years since they can be applied to liquid crystal display devices of the active matrix type, image sensors of the closely contacting type and various other applications. Attention is paid particularly to polycrystalline silicon as a thin film material since thin film transistors constituting a peripheral driver circuit can be formed on a same substrate of an integrated circuit in addition to thin film transistors constituting a display section and a sensor section.
For thin film transistors which are used as switching elements for switching picture elements of a liquid crystal display device of the active matrix type on and off, various structures have conventionally been proposed and put into practical use particularly in order to minimize the leak current which causes a defect of a bright point of a picture element. For example, a thin film transistor of a lightly doped drain structure (LDD) has been developed and is disclosed in Japanese Patent Publication Application No. Heisei 3-38755. The thin film transistor of the lightly doped drain structure has, between end portions of a channel region and a drain region, a low density impurity region thinner than the drain region. The lightly doped drain structure is applied to circuit elements of a liquid crystal display device of the active matrix type and like devices since it is advantageous in that the concentration of an electric field at the end portion of the drain region can be alleviated and it has a leak current suppressing effect.
In order to make the background of the present invention clear, a conventional lightly doped drain structure will be described briefly with reference to FIG. 12. A polycrystalline silicon film is formed in an island-like pattern on the surface of a quartz substrate 1. The polycrystalline silicon film has a channel region 2, a source region 3, and a drain region 5 formed on the opposite sides of the channel region 2. Such a low density impurity region or lightly doped drain region 6 as described above is formed at each of the opposite end portions of the channel region 2. A gate electrode 9 is formed in a pattern on the polycrystalline silicon film with a gate oxide film 7 and a gate nitride film 8 interposed therebetween, thereby constituting a thin film transistor. A first interlayer insulator film 10 is formed on the thin film transistor. A wiring electrode 11 is formed in a pattern on the first interlayer insulator film 10 and is electrically connected to the source region 3 by way of a contact hole. A picture element electrode 13 is formed in a pattern on the thin film transistor with a second interlayer insulator film 12 interposed therebetween and is electrically connected to the drain region 5 by way of a contact hole.
A thin film transistor of a so-called multi-gate structure which includes two or more gate electrodes is conventionally known as another measure to minimize the leak current of a thin film transistor. Thin film transistors of the type mentioned are disclosed, for example, in Japanese Patent Laid-Open Application No. Showa 58-171860 and Japanese Patent Laid-Open Application No. Showa 58-180063. In order to facilitate understanding of the present invention, a thin film transistor of the multi-gate structure will be described briefly with reference to FIG. 13. A polycrystalline silicon film is formed in an island-like pattern on the surface of a quartz substrate 1. The polycrystalline silicon film has a pair of channel regions 2 formed in a mutually separated condition from each other therein. The channel regions 2 are connected to each other by way of a source/drain region 4. A source region 3 is formed at an end portion of one of the channel regions 2 remote from the source/drain region 4 while a drain region 5 is formed at an end portion of the other channel region 2 remote from the source/drain region 4. A pair of gate electrodes 9 are formed in a predetermined pattern in register with the channel regions 2 with a gate oxide film 7 interposed therebetween. A wiring electrode 11 is formed in a pattern on the polycrystalline silicon film with a first interlayer insulator film 10 interposed therebetween and is electrically connected to the source region 3 by way of a contact hole formed in the first interlayer insulator film 10. A picture electrode 13 is formed in a pattern on the polycrystalline silicon film with a second interlayer insulator film 12 interposed therebetween and is electrically connected to the drain region 5 by way of another contact hole. The multi-gate thin film transistor thus includes, represented in an equivalent circuit, a plurality of thin film transistors connected in series. Since the leak current depends upon that one of the thin film transistors which is lowest in off-current value, the leak current can be suppressed. Therefore, thin film transistors of the multi-gate structure are also applied to switching elements for picture elements of a liquid crystal display device of the active matrix type or a like device.
FIG. 14 is an equivalent circuit diagram showing a portion for a picture element of a liquid crystal display device of the active matrix type which employs thin film transistors of the multi-gate structure. Referring to FIG. 14, the liquid crystal display device includes switching elements TFT1 to TFTn connected in series. The gate electrodes of the switching elements TFT1 to TFTn are connected commonly to a gate line. An end portion of the source region of the thin thin film transistor TFT1 is connected to a signal line while an end portion of the drain region of the thin film transistor TFTn drives a liquid crystal display element by way of a picture element electrode. An auxiliary capacitor is connected in parallel to the liquid crystal display element.
In the conventional lightly doped drain thin film transistor structure, since the dose amount of an impurity in the lightly doped drain region is approximately 1.times.10.sup.12 to 1.times.10.sup.13 /cm.sup.2, when ions of an impurity are implanted into the polycrystalline silicon film, the specific resistance value of the polycrystalline silicon film is varied by a great amount by a small variation of the dose amount. Consequently, the resistance of the lightly doped drain region is liable to vary, which is a cause of a dispersion in the leak current of lightly doped drain thin film transistors. In a thin film transistor whose leak current is high, particularly when the ambient temperature is high, for example, 50.degree. to 80.degree. C., the leak current increases exponentially with respect to the temperature. Consequently, there is a subject to be solved in that a so-called high temperature bright point defect appears on the screen of the liquid crystal display device. Further, since the activation ratio of impurity ions in an active region is varied by a small difference in the crystalline property of a polycrystalline silicon film used for an active region of a thin film transistor, there is another subject in that the threshold voltage (Vth) varies among thin film transistors. In addition, since thin film transistors have a dispersion in capacitance coupling between the gate capacitance and the auxiliary capacitance, bright line defects in the form of a thin string sometimes appear on the screen of a liquid crystal display device of the active matrix type, which is a further subject to be solved. Such bright line defects arising from the capacitance coupling appear significantly particularly when a signal charge is written into picture element electrodes while the drain voltage is comparatively low.
On the other hand, in the conventional thin film transistor of the multi-gate structure, since the impurity doped in the source region and the drain region is diffused in a horizontal direction, there is a subject to be solved in that the length of the channel, for example, of an n-channel thin film transistor in which p.sup.+ ions are doped cannot be reduced smaller than 5 .mu.m. If the channel length is reduced in this manner, then the effective channel length becomes excessively short due to the dispersion of the impurity in a horizontal direction, and consequently, the leak current increases remarkably. Therefore, miniaturization of thin film transistors is difficult with the conventional multi-gate structure, which is an obstacle to enhancement of the resolution of a liquid crystal display device of the active matrix type. In addition, a liquid crystal display device in which thin film transistors of the conventional multi-gate structure are employed suffers very frequently from high temperature bright point defects similarly to a liquid crystal display device in which thin film transistors of the lightly doped drain structure are employed.
Furthermore, even where a thin film transistor has the lightly doped drain structure, it cannot completely suppress the occurrence of a bright point defect picture element caused by leakage of signal charge from the thin film transistor during holding of the signal charge. Particularly at a high temperature, for example, 55.degree. C., at which the leak level is generally high, point defect picture elements which are put into a blinking condition appear very frequently. Thus, an analysis has been made of the phenomenon. The analysis has proved that point defects of picture elements relate to ac driving of the liquid crystal and depend upon the polarity of the applied voltage and leak current flows between the source and the drain. More particularly, during holding of signal charge of the positive polarity, a high electric field is applied continuously between the gate electrode and the picture element electrode, and consequently, the leak current flowing associated with a localized state is much higher than that during holding of a signal charge of the negative polarity so that the image signal level of a high potential cannot be held sufficiently during holding of the positive polarity, resulting in loss of the balance of ac driving of the liquid crystal. Consequently, there is a subject to be solved in that blinking point defect picture elements appear very frequently.